A catalytic cracking catalyst is recycled between a reactor and a regenerator. Generally, upon leaving the reactor, the catalyst contains about 3-10 wt % of coke. The catalyst should be burned with oxygen in air in the regenerator to remove the deposited coke to recover its catalytic activity. The coke deposited in/on the catalyst is mainly a condensation reaction product, which is substantially composed of carbon and hydrogen. In case that the cracking feedstock contains sulfur and nitrogen, the coke also contains sulfur and nitrogen. The coked catalyst is regenerated with oxygen to produce CO2, CO and H2O, and the regeneration flue gas further contains SOx (SO2, SO3) and NOx (NO, NO2). The regeneration reaction is an exothermal reaction and has a large thermal effect that is sufficient to provide a heat quantity required by the heat balance of the catalytic cracking plant. The flue gas leaving the regenerator of the catalytic cracking plant has a temperature of 620-690° C. and a pressure of 0.1-0.25 MPa, and therefore has a higher energy grade. Generally, in order to facilitate recovering and reusing the energy of the flue gas, a flue gas turbine expander is used to produce electricity, then a waste heat boiler is used to produce steam, and finally the blowdown gas has a temperature of 170-250° C. and a pressure of a very small positive pressure or even a negative pressure. If the regenerator is operated in an oxygen-deficient manner, the content of carbon monoxide will be relatively higher, in order to avoid the tail gas combustion, a carbon monoxide combustion aid is needed and an additional oxygen is supplied so that carbon monoxide can be converted to carbon dioxide.
In the other hand, the hydrocarbon feedstock for the catalytic cracking plant is subjected to a cracking reaction and a fractionation to produce a variety of fractions, wherein the gaseous fractions include a dry gas fraction. The dry gas contains H1, small molecule hydrocarbon and non-hydrocarbon compounds, H2S and the like. Generally, the dry gas should be subjected to an H2S removal step before it is utilized. The removed H2S is further passed through a means such as Claus to be converted to an elemental sulfur (the elemental sulfur has a boiling point of 445° C. and a melting point of 113° C.) and the recovery is eventually completed.
The sulfur carried from the feedstock into the catalytic cracking plant is present in an oxidation state in the flue gas and in a reduction state in the dry gas. According to the existing technologies, the removal of sulfur oxides in the flue gas and the removal of H2S in the dry gas are separately conducted. The technology of simultaneously treating the flue gas and the dry gas is seldom, if not never, reported up to now.